For propellants generally, the art has recognized that incorporation of finely divided boron, aluminum, or other metals in a propellant formulation often increases its burning rate and specific impulse. See: U.S. Pat. No. 3,196,059, issued to Godfrey on Jul. 20, 1965, column 7, starting on line 47. All the examples of Godfrey use 20% or more aluminum.
U.S. Pat. No. 2,995,431, issued to Bice on Aug. 8, 1961, teaches a composition of 2 to 6 parts (from about 2% to about 6%) boron, aluminum, or magnesium, 8 parts binder, 90 parts (about 88%) ammonium nitrate, and 2 parts catalyst (Note: all parts, portions and percentages herein are by weight unless otherwise indicated.)
A number of other patents also broadly suggest the use of a wide range of proportions of a finely divided metal in propellants. U.S. Pat. No. 3,423,256 issued to Griffith on Jan. 21, 1969, column 5, lines 51-60, teaches the utility of adding 0.5 to 30%, preferably 0.5 to 25%, of a fuel such as finely divided aluminum to a propellant composition. In Examples 13 and 14 of the reference, a composition containing 5% aluminum in combination with ammonium nitrate, a nitrate ester, and other ingredients is disclosed. U.S. Pat. No. 3,465,675, issued to Bronstein, Jr. on Sept. 9, 1969, teaches an explosive. At column 5, lines 14-33 and Example 3, Bronstein indicates the desirability of adding from 0.5 to 50%, preferably 0.5 to 40%, of a metal fuel such as aluminum or boron. Reissue Patent 25,695, reissued to Cook, et al. on Dec. 8, 1964, teaches an explosive which contains aluminum and ammonium nitrate in a slurry with water. Column 5, lines 30-57, indicates that the compositions can comprise a mixture of 5 to 25 % trinitrotoluene (TNT) and 75 to 95% of a mixture of 60 to 100 parts ammonium nitrate and 0 to 40 parts fine aluminum. At column 6, lines 11-13, Cook indicates that boron and magnesium function as well as aluminum, but are more expensive.
The prior art has stated that boron does not increase the burning rate of a propellant under certain circumstances, however. The previously cited Bice patent, in column 10, lines 67-75 and column 11, lines 13-24, states that for a composition containing 82.5% ammonium nitrate, "the addition of boron is actually detrimental to the burning rate and burning rate exponent". Other examples in Bice show that, in the presence of more than 86% ammonium nitrate, the addition of boron increases the burning rate and pressure exponent. (It is desirable, however, to reduce the pressure exponent.)
The previously cited Bice patent illustrates the use of a large proportion of ammonium nitrate (at least 86% by weight) along with minor amounts of a polymeric binder, boron, aluminum, other metals, and other ingredients in a propellant grain. One problem in the art represented by the Bice patent is that propellants which contain more than about 75% ammonium nitrate are difficult and expensive to fabricate into propellant grains. The Bice patent teaches the need to compression mold the propellant at pressures on the order of 3000 to 20,000 psi (about 2100 to about 14,000 Newtons per centimeter squared; abbreviated "N/cm.sup.2 "). The uncured Bice propellants thus are not pourable slurries, and cannot be simply cast into a motor casing and cured in situ to form a propellant grain. An oversized grain must be formed in a pressure mold and machined to provide a grain of the desired dimensions.
Another continuing problem in the art is how to provide a propellant grain which has a low pressure exponent (ideally zero) when burned at a high pressure. The variation of burning rate with pressure is commonly expressed by a burning rate equation as follows: EQU r=ap.sup.n
wherein "r" represents the burning rate, "a" is a variable which depends on the initial grain temperature, and "p" is the pressure in the combustion chamber. The value of "n", the pressure exponent, should be as close as possible to zero over the range of pressures for which the rocket motor is designed. If "n" is uniformly positive, the burn rate will be unstable because a rise in pressure will increase the burn rate, which will in turn increase the pressure. If this positive feedback is substantial, the rocket will over-pressurize and may explode. One problem in the art has been that if a grain having a relatively high operating pressure range is desired, it is difficult to provide a propellant which has a zero pressure exponent.
Ballistic modifiers are propellant ingredients which lower the pressure exponent of a propellant over a certain range of combustion pressures. Ideally, a ballistic modifier would make the pressure exponent zero at all pressures likely to be encountered in the combustion chamber, thus providing a constant burning rate. However, in the real world, a pressure exponent of zero can be approximated only over a fairly narrow range of operating pressures. Typically, above and below the pressure range in which the ballistic modifier operates, the pressure exponent is positive.
Furthermore, the effect of a ballistic modifier in a particular formulation is frequently unpredictable. An ingredient which is an effective ballistic modifier with one binder, or one curing agent, or one distribution of oxidizer particle sizes, or in one proportion, may not be an effective ballistic modifier when one of these parameters is changed. If the characteristic combustion temperature of a propellant is changed substantially, the effect of the ballistic modifier can be changed or even eliminated in some cases. One side effect of excessive use of many ballistic modifiers is to decrease the burning rate of the propellant at all pressures, which is undesirable. Therefore, it is necessary to tailor a particular propellant formulation by trial and error to provide the desired pressure exponent, burning temperature, burning rate, and other properties simultaneously.